Jiann-Chang Lin

A theoretical study on Bunsen spray flames

Abstract The structure of Bunsen flame tip under the influence of dilute, monodisperse inert (water) or fuel (methanol) sprays is theoretically studied using large activation energy asymptotics. A completely prevaporized mode is identified, in which no liquid droplets exist downstream of the flame. Parameters for open and closed flame tips in the analysis consist of the amount of liquid loading indicating the internal heat loss for the water spray or the internal heat loss and heat gain for the rich and lean methanol-sprays, respectively, and the (negative) stretch coupled with Lewis number ( Le ) which strengthens the burning intensity of the Le >1 flame but weakens that of the Le Le >1) with water sprays (or lean methanol-spray flames with Le >1), closed-tip solutions are obtained. The burning intensity of the flame tip is enhanced with either decreasing liquid-water loading (or increasing liquid-fuel loading) or increasing stretch. Conversely, the negative stretch weakens the burning intensity of a lean methane–air flame ( Le Le Le >1) and thus leads to the transition of flame configurations from conventional Bunsen cone through planar flame to inverted flame cone (a convex flame shape with respect to the upstream reactants). The critical value of liquid-fuel loading, at which there exists a planar flame rather than a Bunsen cone flame, is increased with either increasing upstream flow velocity or decreasing equivalence ratio.

Influence of water spray on normal and inverted bunsen flame

The structure of normal and inverted Bunsen flame tips under the influence of preferential diffusion, flame stretch, and inert (water) spray is theoretically studied using large-activation-energy asymptotics. A completely prevaporized mode and a partially prevaporized mode are identified. Analytic parameters for open and closed flame tips consist of amount of liquid loading and initial droplet size, indicating internal heat loss associated with liquid water vaporization, and negative (or positive) stretch coupled with Lewis number ( Le ), strengthening (or decreasing) and weakening (or increasing) burning intensity of the Le >1 and Le 1) and a lean methane/air and rich propane/air invertedBunsenflame( Le <1),closed-tip solutions are obtained. Burning intensity increases toward the flame tip, which has the largest curvature, and is enhanced with decreasing liquid-water loading, increasing initial droplet size, or increasing stretch. Conversely, stretch weakens burning intensity of the lean methane/air and rich propane/air normal Bunsen flame ( Le <1), and the rich methane/air and lean propane/airinverted Bunsen flame ( Le >1), eventually leading to tip opening; that is, flame extinction. Burning intensity is further reduced with increasing liquid-water loading, increasing stretch, or decreasing initial droplet size. In addition, the opening becomes wider for an open-tip normal and inverted Bunsen flame when liquid-water loading or upstream flow velocity increases or initial droplet size decreases.

The influence of preferential diffusion and stretch on the burning intensity of a curved flame front with fuel spray

Abstract In our most recent paper on Bunsen spray flames, only a completely prevaporized mode of a normal Bunsen flame was considered; inverted Bunsen flame and droplet size effects had not been examined yet. In the present study, we consider two flame structures: normal and inverted Bunsen flames, and two spray modes: completely and partially prevaporized burning, by the method of large activation energy asymptotics. In this way, a complete parametric study of flame tip intensification or extinction (opening) can be conducted. Four parameters are used in the analysis. The first two are the droplet size and amount of liquid-fuel loading, which indicate internal heat loss for a rich spray but heat gain for a lean spray. The other two are the stretch and Lewis number (Le). Stretch is negative for a normal Bunsen flame but positive for an inverted Bunsen flame. Stretch strengthens (or weakens) the burning intensity of the Le>1 (or Le 1 (or Le 1 or a rich methanol-spray inverted Bunsen flame with Le 1, or rich methanol-spray normal Bunsen flames with Le 1, if liquid loading is large enough and droplet size is sufficiently small, there exists flame transition from normal (or inverted) Bunsen through planar to inverted cone (or normal Bunsen) flame. Finally, the critical value of droplet size, at which there exists a planar flame rather than a normal (or an inverted) Bunsen flame, increases with increasing liquid loading.